A known spark plug for use in an internal combustion engine includes a metallic shell having a tool engagement portion and a mounting thread formed thereon, and a ceramic insulator (insulator) inserted into a through hole which extends through the metallic shell in the axial direction. Such a spark plug is configured such that spark discharges are generated between a forward end portion of a center electrode attached to the ceramic insulator and a ground electrode joined to a forward end portion of the metallic shell.